The Redpark Serial Cable for iOS is the first and only cable approved by Apple for connecting your iOS device to almost anything using serial communication! With this cable, you can open up your iPhone, iPad, or iPod touch and let your imagination run wild! Use external sensors in your iOS apps! Talk to an Arduino with your iPhone! The possibilities are unlimited!

and the new ADK Sensor Kit (it’s $359, but it includes huge selection of sensors, an Arduino Mega ADK, and sensor shield for plugging in all those sensors):

Early models
describing the kinematics of an individual target joint during the
various phases of manipulation (notably Sandoz 1976) were based on
studies that investigated joint cracking in MCP joints.